One type of electrical apparatus that has a sealed fluid based insulating system is an electrical property measuring and field controlling apparatus, which may be a bushing. Bushings may be provided for pieces of electrical high-voltage equipment and are known to carry current of high voltage through a zone of a different potential, which different potential is ground potential when the bushing is a transformer bushing. In order to avoid being a safety hazard, the bushing itself needs to be monitored regarding how well it is functioning.
One way to perform this may be through opening the bushing and take samples of the fluid, for instance oil, which is used as an insulating medium inside the bushing. The temperature of this oil can then be measured and used for diagnosing the bushing. However, when this is done the properties of the bushing are altered. The amount of oil in the bushing may be changed and the oil may also become contaminated when opening the bushing. There is in this case also a risk that the bushing is not sealed properly after such taking of samples.
There is therefore a need for an alternative way of diagnosing a bushing.
There are in the art some documents describing diagnosing of transformers.
US 2005/0223782 does for instance describe a transformer monitoring system. Here the current of the transformer and an ambient temperature are measured. These are used for determining an oil temperature of the transformer. The actual transformer oil temperature is then measured and compared with the determined oil temperature and an anomaly is determined if a temperature difference exceeds a threshold. Also the inner pressure of the transformer is determined and compared with a threshold and an anomaly is determined if this pressure exceeds a pressure threshold.
US 2002/0161558 describes a transformer monitoring system. In the system a number of physical quantities are monitored, like current, top oil temperature and ambient temperature.
U.S. Pat. No. 4,654,806 describes a transformer monitoring system. In this system various parameters are measured, such as oil temperature, ambient temperature and pressure. These are then each compared with a corresponding threshold.
All of these documents use monitoring of a transformer top oil temperature.
There do also exist some documents related to diagnosing bushings.
EP 747 715 does for instance describe diagnosing the status of a bushing through measuring imbalance current waveforms from a number of bushings, comparing imbalance values with each other and initial values to determine if a threshold is exceeded. If the threshold is exceeded there is a determination being made about which of the bushings that have changed as well as the changes in capacitances and power values of these bushings. These changed capacitances and power values are then temperature compensated based on (transformer) top-oil and ambient temperatures. This document seems to require several bushings for performing diagnosing and furthermore only measures the current.
The article “New Insulation Diagnostic and Monitoring Techniques for In-Service HV Apparatus” BY D. M. Allan, M. S. Blundell, K. J. Boyd and D. D. Hinde, Proceedings of the 3rd International Conference on Properties and Applications of Dielectric Materials, Jul. 8-12, 1991, Tokyo, Japan describes a monitoring system for a transformer as well as for a transformer bushing through monitoring DDF (Dielectric Dissipation Factor).
There is thus a need for diagnosing an electrical apparatus having a sealed fluid based insulating system that can be used without opening the electrical apparatus and taking samples and that may be applied for one single electrical apparatus.